The broad objectives of this research are to investigate the properties of living human photoreceptors. The properties under investigation include: (a) the directional sensitivity of the retina (Stiles-Crawford effect); (b) the optical aperture of the cones; (c) the distribution of orientations of the cones (averseness); (d) the long-term effects of the direction of incident light on the orientations of cones (phototropism); (f) the processes of light adaptation and dark adaptation within cones; (g) the interactions between rods and cones (rod-cone interactions). In order to minimize the effects of the techniques of observations upon the properties observed, psychophysical techniques, consisting mainly of determinations of thresholds, will be used.